The present invention relates to an endless metal belt for a continuously variable transmission for a vehicle, having a layered ring structure formed by superposing a plurality of thin metal rings in layers so as to be slidable relative to each other and a plurality of metal blocks supported on the layered ring structure so as to be slidable relative to the layered ring structure, and extended between a pair of pulleys for torque transmission.
A drive belt, i.e., an endless metal belt of this kind, disclosed in, for example, JP-B-Hei 2-22254 has a carrier, which corresponds to the layered ring structure, formed by superposing a plurality of endless metal bands, which correspond to the thin metal rings, and a plurality of cross members, which corresponds to the metal blocks, combined with the carrier so as to be slidable relative to the carrier. The inner circumference of each metal band is provided with intersecting grooves to reduce friction between the adjacent metal bands that slide relative to each other in order to improve the torque transmission efficiency of the drive belt.
When the carrier is tensioned, a greater pressure is produced in the inner endless metal band, and a pressure applied to the cross members by the innermost endless metal band is the greatest. When the innermost endless metal band and the cross members slide relative to each other in a state where the maximum pressure is applied to the cross members by the innermost endless metal band, friction is more liable to be produced between the innermost endless metal band and the cross members than between the adjacent endless metal bands. Since the endless metal bands are thin and a high tension is induced in the endless metal bands, the abrasion of the inner circumference of the innermost endless metal band is one of the factors that dominate the durability of the carrier, and hence that of the drive belt.
The innermost endless metal band of the carrier of the aforesaid prior art drive belt is provided in its inner circumference with the intersecting grooves, and the inner circumference of the innermost endless metal band is in sliding contact with the upper surfaces of the cross members. It was found that the inner circumference of the innermost endless metal band is abraded rapidly when the inner circumference of the innermost endless metal band that applies the maximum pressure to the cross members is provided with the grooves as mentioned above.
FIG. 5 is a graph showing data obtained through experiments using an endless metal belt type continuously variable transmission driven by an internal combustion engine. In the experiments, input torque was 161.8 Nm, engine speed was 6000 rpm and speed change ratio was 0.61 (overdrive speed). FIG. 5 shows the change with time of the maximum peak-to-valley height Rmax, i.e., an index of the surface roughness, of the circumference of the endless metal band. FIG. 6 is a graph obtained on the basis of the data shown in FIG. 5, showing the relation between the surface roughness (Rmax) of the of the endless metal band and the surface roughness change rate (Rmax change rate). It is obvious from those graphs that the inner circumference of the endless metal band provided with the grooves is abraded rapidly because the depth of the grooves formed in the same inner circumference is in the range of 2.5 to 5.0 xcexcm. It is known from those graphs that the progress of abrasion becomes very slow when the surface roughness Rmax is 2.0 xcexcm or below.
When the endless metal band is subjected to heat treatment or a surface hardening process, such as a nitriding process, after forming the grooves in the circumference of the endless metal band, a hard skin LH (FIG. 8) and an internally stressed layer having a residual stress are formed in an inner circumferential part provided with the grooves of the endless metal band.
FIG. 7 is a graph shows the variation of the Vickers hardness HV of the endless metal band with distance from the inner circumference in which the hard skin LH is formed. It is known from this graph that the rate of abrasion of the endless metal band increases after the hard skin LH has been removed by abrasion and a part having a low hardness has been exposed. The inner circumference of the endless metal band provided with the intersecting grooves (meshy grooves) has both hard parts and soft parts as shown in FIG. 8 when the inner circumference of the endless metal band is abraded, and the inner circumference having such an irregular hardness is abraded more rapidly than the inner circumference having the uniform hardness. In FIG. 8, the width C1 of ridges defining the grooves and the depth C2 of the grooves are exaggerated and the proportion between C1 and C2 does not represents an actual proportion. In FIG. 8, two-dot chain lines indicate the inner circumference before abrasion.
FIG. 9 is a graph showing variation of the residual stress in the internally stressed layer with distance from the inner circumference of the endless metal band. It is known from this graph that the fatigue strength of the inner circumference of the innermost endless metal band that undergoes repeated load decreases, the rate of abrasion increases because a part having a low internal stress is exposed when the inner circumference is abraded progressively. As the inner circumference provided with the intersecting grooves of the endless metal band of the drive belt is abraded progressively, both parts having a high residual stress and those having a low residual stress appear in the inner circumference. Consequently, the inner circumference is irregularly stressed and the fatigue strength thereof decreases.
Thus the innermost endless metal band of the prior art drive belt is abraded markedly, and the carrier, hence the drive belt, needs improvements relating with durability.
The present invention has been made in view of such problems and it is therefore an object of the present invention to provide a durable layered ring structure for an endless metal belt.
According to the present invention, an endless metal belt to be extended between a pair of pulleys to transmit torque from one of the pulleys to the other comprises: a pair of layered ring structures each formed by superposing a plurality of thin metal rings in layers so as to be slidable relative to each other; and a plurality of metal blocks arranged along the length of the pair of layered ring structures and supported on the pair of layered ring structures so as to be slidable relative to the layered ring structures; wherein at least one of the circumferences in sliding contact with each other of adjacent thin metal rings is provided with oil retaining grooves, and the inner circumference in sliding contact with the saddle surfaces of the metal blocks of the innermost thin metal ring of each layered ring structure is a flat, smooth surface which is free from any oil retaining grooves.
The circumferences of the adjacent thin metal rings of the layered ring structure are lubricated with a lubricating oil retained in the oil retaining grooves and hence those circumferences are not abraded easily. Since the inner circumference in contact with the saddle surfaces of the metal blocks of the innermost thin metal ring is formed in a flat, smooth surface of a low surface roughness, the inner circumference is not abraded easily. Since the inner circumference of the innermost thin metal ring is free from or is not provided with any oil retaining grooves, formation of parts respectively having different hardnesses and those having different internal stresses in the inner circumference of the innermost thin metal ring due to abrasion can be prevented and hence the abrasion of the inner circumference of the innermost thin metal ring can be suppressed. Consequently, the hardness and the residual stress of the surface layer of the inner circumference of the innermost thin metal ring can be maintained for a long period of time.
The abrasion of the circumferences of the thin metal rings is suppressed by the lubricating effect of the lubricating oil retained in the oil retaining grooves formed in the circumferences of the thin metal rings. The flat inner circumference in sliding contact with the saddle surfaces of the metal blocks of the innermost thin metal ring is not abraded easily. Thus the durability of the layered ring structure of the endless metal belt can be enhanced and thereby the durability of the endless metal band can be enhanced.
Preferably, the flat inner circumference of the innermost thin metal ring is finished in a surface roughness Rmax of 2.0 xcexcm or below.
Since the inner circumference of the innermost thin metal ring is finished in such a small surface roughness, the progress of abrasion of the inner circumference can be highly effectively retarded, which further enhances the durability of the layered ring structure.
Preferably, the saddle surfaces of the metal block are finished in a surface roughness Rmax in the range of 2.0 to 4.0 xcexcm
Thus friction coefficient between the inner circumference of the innermost thin metal ring and the saddle surface of each metal block is smaller than that between the inner circumference provided with the intersecting grooves of the prior art endless metal band and the metal block. Thus allowable input torque can be increased, suppressing abrasion. Since both the inner circumference of the innermost thin metal ring and the saddle surfaces of the metal blocks are not provided with any grooves, bearing pressures acting on the inner circumference and the saddle surfaces are low and hence the inner circumference of the innermost thin metal ring is not abraded rapidly.
Thus, the abrasion of the inner circumference of the innermost thin metal ring can be further reduced, the durability of the layered ring structure is further enhanced, the friction coefficient between the inner circumference of the innermost thin metal ring and the saddle surfaces of the metal blocks is reduced and, consequently, allowable input torque can be increased, suppressing abrasion.